Dipolar-octupolar correlations and hierarchy of exchange interactions in Ce$_2$Hf$_2$O$_7$

Abstract: We investigate the correlated state of Ce$_2$Hf$_2$O$_7$ using neutron scattering, finding signatures of correlations of both dipolar and octupolar character. A dipolar inelastic signal is also observed, as expected for spinons in a quantum spin ice (QSI). Fits of thermodynamic data using exact diagonalization methods indicate that the largest interaction is an octupolar exchange, with a strength roughly twice as large as other terms. A hierarchy of exchange interactions with dominant octupolar and significant dipolar exchange, still in the octupolar QSI phase, rationalises neutron scattering observations. Our results reveal a `quantum multipolar liquid' where correlations involve multiple terms in moment series expansion, opening questions about their intertwining and possible hierarchy.